Biglycan, a small leucine-rich proteoglycan (SLRP), is involved in collagen fibrillogenesis and also acts as a signaling molecule. Although decorin has been considered as the primary SLRP in developing and maintaining tendon structure and mechanics, more recent work using inducible knockdown models suggests that biglycan is involved in tendon homeostasis. The purpose of the study was to determine the role of biglycan in tendon homeostasis to maintain mechanical and structural integrity in aged mice. Aged (485 days old) female Bgn^+/+ control (wild type [WT], n = 16) and 16 bitransgenic conditional Bgn^flox/flox mice (I-Bgn^−/−, n = 16) with a tamoxifen-inducible Cre (driven by ROSA) were utilized. After biglycan knockdown, the transgenic model demonstrated effective knockdown of the target gene without any compensation from other SLRPs or type I collagen. Patellar tendon cellularity was not modified after biglycan knockdown. However, biglycan knockdown had an impact on collagen fibrillogenesis with a higher percentage of small diameter fibrils (25–45 nm) and a lower percentage of medium size fibrils (150–165 nm) in I-Bgn^−/− tendons. Biglycan knockdown also induced a reduction in the midsubstance modulus and maximum stress compared to WT. Stress relaxation was reduced at 4% strain in I-Bgn^−/− tendons but no changes were observed in dynamic modulus and tan delta. As in mature tendons (120 days old), this study showed significant effects of biglycan knockdown on mechanical and structural properties of aged tendons only 30 days after knockdown. These data suggest that biglycan has a major role in maintaining homeostasis in aged tendon.